In general, to efficiently remove harmful components of exhaust gas for purification using a catalyst, an internal combustion engine with an exhaust purification system utilizing the catalyst needs to control the mixing ratio between air and fuel in an air-fuel mixture combusted in the internal combustion engine, that is, the air-fuel ratio. For such control of the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine to perform feedback control to make the detected air-fuel ratio equal to a predetermined target air-fuel ratio.
On the other hand, a multicylinder internal combustion engine normally controls the air-fuel ratio using identical control amount for all cylinders. Thus, even when the air-fuel ratio control is performed, the actual air-fuel ratio may vary among the cylinders. In this case, if the variation is at a low level, the variation can be absorbed by the air-fuel ratio feedback control, and the catalyst also serves to remove harmful components of exhaust gas for purification. Consequently, such a low-level variation would not affect exhaust emissions and pose an obvious problem.
However, if, for example, fuel injection systems for some cylinders become defective to significantly vary the air-fuel ratio among the cylinders, the exhaust emissions disadvantageously deteriorate. Such a significant variation in air-fuel ratio as deteriorates the exhaust emissions is desirably detected as abnormality. In particular, for automotive internal combustion engines, there has been a demand to detect inter-cylinder air-fuel ratio imbalance in a vehicle mounted state (on board) in order to prevent a vehicle with deteriorated exhaust emissions from travelling. There has recently been a trend to legally regulate the detection of the inter-cylinder air-fuel ratio imbalance.
For example, in an apparatus described in PTL 1, the inter-cylinder air-fuel ratio imbalance is detected in a driving system including a turbocharger, a bypass passage that bypasses a turbine in the turbocharger, and a waste gate valve that opens and closes the bypass passage. The detection is performed using an air-fuel ratio sensor disposed in a portion of an exhaust passage on a downstream side of a junction between a downstream side of the turbine and a downstream side of the bypass passage. With focus placed on the fact that the influence of inter-cylinder imbalance of the air-fuel ratio is likely to appear in exhaust gas having passed through the bypass passage, the driving system detects the inter-cylinder air-fuel ratio imbalance based on an output from the air-fuel ratio sensor while the waste gate valve is open.
PTL 2 notes that, in a similar mechanical configuration, when the air-fuel ratio imbalance detection is performed while the waste gate valve is closed, the accuracy of the air-fuel ratio imbalance detection is deteriorated due to the adverse effect of stirring of exhaust gas by the turbine. To avoid this, the apparatus disclosed by PTL 2 changes an operating line (optimal fuel efficiency line) so that, when an operating point is present outside an “open” region of a waste gate valve (in a coordinate system defined by engine rotational speed and torque), the actual operating point moves into the “open” region, and controls the engine and an automatic transmission in accordance with the changed operating line. That is, the apparatus in PTL 2 performs the imbalance detection only while the waste gate valve is open.